Nematodes are elongated, symmetric roundworms. Parasitic nematodes of medical significance may be broadly classified as either predominantly intestinal or tissue nematodes. This chapter covers the tissue nematodes that cause trichinellosis, visceral and ocular larva migrans, cutaneous larva migrans, cerebral angiostrongyliasis, and gnathostomiasis. All of these zoonotic infections result from incidental exposure to infectious nematodes. The clinical symptoms of these infections are due largely to invasive larval stages that (except in the case of Trichinella) do not reach maturity in humans.
Trichinellosis develops after the ingestion of meat containing cysts of Trichinella (e.g., pork or other meat from a carnivore). Although most infections are mild and asymptomatic, heavy infections can cause severe enteritis, periorbital edema, myositis, and (infrequently) death.
Life Cycle and Epidemiology
Eight species of Trichinella are recognized as causes of infection in humans. Two species are distributed worldwide: T. spiralis, which is found in a great variety of carnivorous and omnivorous animals, and T. pseudospiralis, which is found in mammals and birds. T. nativa is present in Arctic regions and infects bears; T. nelsoni is found in equatorial eastern Africa, where it is common among felid predators and scavengers such as hyenas and bush pigs; and T. britovi is found in Europe, western Africa, and western Asia among carnivores but not among domestic swine. T. murrelli is present in North American game animals.
After human consumption of trichinous meat, encysted larvae are liberated by digestive acid and proteases (Fig. 216-1). The larvae invade the small-bowel mucosa and mature into adult worms. After ∼1 week, female worms release newborn larvae that migrate via the circulation to striated muscle. The larvae of all species except T. pseudospiralis, T. papuae, and T. zimbabwensis then encyst by inducing a radical transformation in the muscle cell architecture. Although host immune responses may help to expel intestinal adult worms, they have little effect on muscle-dwelling larvae.
Life cycle of Trichinella spiralis (cosmopolitan); nelsoni (equatorial Africa); britovi (Europe, western Africa, western Asia); nativa (Arctic); murrelli (North America); papuae (Papua New Guinea); zimbabwensis (Tanzania); and pseudospiralis (cosmopolitan). CNS, central nervous system. [Reprinted from Guerrant RL et al (eds): Tropical Infectious Diseases: Principles, Pathogens and Practice, 2nd ed, p 1218. © 2006, with permission from Elsevier Science.]
Human trichinellosis is often caused by the ingestion of infected pork products and thus can occur in almost any location where the meat of domestic or wild swine is eaten. Human trichinellosis also may be acquired from the meat of other animals, including dogs (in parts of Asia and Africa), horses (in Italy and France), and bears and walruses (in northern regions). Although cattle (being herbivores) are not natural hosts of Trichinella, beef has been implicated in outbreaks when contaminated or adulterated with trichinous pork. ...